1. Field of the Invention
The present invention relates to power amplifiers. More specifically, the present invention relates to power amplifiers with predistorters.
2. Description of the Related Art
The operating range of an amplifier is characterized by a linear region where the gain, the ratio of the output power to the input power, is substantially constant and a nonlinear region where the gain varies. In the nonlinear region, the gain usually decreases as input power increases and is usually referred to as the compression region. If the input signal is confined to the linear region of the amplifier, the output signal is amplified without appreciable distortion. Conversely, if the input signal spills into the nonlinear region, the output signal will be distorted. The distortion caused by the amplifier depends on the characteristics of the particular amplifier and may be taken into account when designing amplifiers.
In a predistortion system, the amplitude and phase of the signal to be amplified is distorted before amplification such that the output amplified signal is characterized by a substantially constant gain over the range of the input signal. Such a system improves the operating characteristics of the amplifier by compensating and canceling the distortion signal at the input of the amplifier.
The concept of predistortion is well known in the art and is illustratively shown in FIGS. 1 and 2. In FIG. 1, the amplifier gain function 110 is substantially constant below input level, L1, and represents the linear operating range of the amplifier. Above input level, L1, the gain begins to decrease or compress. This compression region corresponds to the nonlinear operating range of the amplifier. In a predistortion system, the input signal is modified by a predistortion function 120, which compensates for the decreasing gain of the amplifier in the nonlinear range such that the total gain function 130 is substantially constant over the linear and nonlinear range of the amplifier.
In FIG. 2, the amplifier phase function 210 is substantially constant below input level, L2, and represents the linear operating range of the amplifier. Above input level L2, the phase of the signal is shifted by a different amount than the shift in the linear range and corresponds to the nonlinear operating range of the amplifier. In a predistortion system, the phase of the input signal is modified by a phase distortion function 220. The phase distortion function 220 compensates for the phase shift difference between the linear and nonlinear ranges of the amplifier such that the total phase function 130 is substantially constant over the linear and nonlinear range of the amplifier.
U.S. Pat. No. 5,172,068, entitled “Third-order Predistortion Circuit”, presents a predistorter for mitigating third-order nonlinearities. The circuit includes first and second branches of series-connected diodes, with the diodes in the first branch being connected in reverse order relative to the diodes in the second branch, resulting in a push-pull arrangement. The diodes may be either pn-junction or Schottky barrier diodes, both of which have exponential transfer functions.
U.S. Pat. No. 5,524,286, entitled “Baseband Predistortion System for the Adaptive Linearization of Power Amplifiers”, discloses a predistortion system based upon the updating of two error tables, one for amplitude and one for phase. The tables' contents are used to correct the baseband samples. The content of the tables is obtained by accumulating the difference, suitably weighted, between the sample entering the predistortion device and the demodulated feedback value.
U.S. Pat. No. 5,589,797, entitled “Low Distortion Amplifier” discloses a low distortion amplifier circuit. The circuit employs a cuber circuit in the predistortion path. The cuber circuit comprises a pair of antiparallel diodes.
U.S. Pat. No. 5,748,678, entitled “Radio Communication Apparatus”, discloses a system in which digital processing in the baseband processor 30 applies a curve-fit routine to the predistortion circuit 28 to predistort the baseband signals.
U.S. Pat. No. 5,929,703, entitled “Method and Device for Modeling AM-AM and AM-PM Characteristics of an Amplifier, And Corresponding Predistortion Method”, develops two series of polynomials respectively representative of the AM-AM and the AM-PM characteristics. The determination of each polynomial allows for the second derivative of the polynomial and for the distances between the samples and points on the curve defined by the polynomial.
U.S. Pat. No. 6,075,411, entitled “Method and Apparatus For Wideband Predistortion Linearization”, discloses the creation of a predistortion signal which is a low order polynomial having adjustable coefficients. The predistortion signal compensates for third order and higher order intermodulation distortion over a wideband, on a coefficient-by coefficient basis.
U.S. Pat. No. 6,107,877, entitled “Method Predistortion Generator Coupled With An RF Amplifier”, depicts Schottky diodes in an antiparallel configuration in a predistorter. The layout of the predistortion circuitry is specifically designed to enhance the performance of the circuitry without inducing any negative operating characteristics on the associated RF amplifier.
The contents of the aforementioned U.S. Pat. No. 5,172,068, U.S. Pat. No. 5,524,286, U.S. Pat. No. 5,589,797, U.S. Pat. No. 5,748,678, U.S. Pat. No. 5,929,703, U.S. Pat. No. 6,075,411, and U.S. Pat. No. 6,107,877 are incorporated by reference to the extent necessary to understand the present invention.